Circuit loads, particularly alternating current (ac) supplied bidirectional resistive elements in lighting or heating devices, can short-circuit during operation. In such cases, switches—particularly bidirectional switches—controlling such loads must be protected.
U.S. Pat. No. 6,975,122, the content of which is incorporated herein by reference, discloses one technique for protecting a bidirectional switch for ac loads. The voltage across the switch may be positive or negative with respect to ground. By attenuating this signal and adding a portion of the supply voltage Vdd, a positive voltage with respect to ground may be detected. While this design improves over prior designs in that only a single polarity power supply is required for the detection circuitry, two comparators are employed: one to detect a possible overcurrent fault in each half wave of the ac power supply.
There is, therefore, a need in the art for improved protection of bidirectional switches.